Micromachined microphone and multisensor and method for producing same

ABSTRACT

A micromachined microphone is formed from a silicon or silicon-on-insulator (SOI) wafer. A fixed sensing electrode for the microphone is formed from a top silicon layer of the wafer. Various polysilicon microphone structures are formed above a front side of the top silicon layer by depositing at least one oxide layer, forming the structures, and then removing a portion of the oxide underlying the structures from a back side of the top silicon layer through trenches formed through the top silicon layer. The trenches allow sound waves to reach the diaphragm from the back side of the top silicon layer. In an SOI wafer, a cavity is formed through a bottom silicon layer and an intermediate oxide layer to expose the trenches for both removing the oxide and allowing the sound waves to reach the diaphragm. An inertial sensor may be formed on the same wafer, with various inertial sensor structures formed at substantially the same time and using substantially the same processes as corresponding microphone structures.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of, and therefore claims priority from,U.S. patent application Ser. No. 11/113,925 filed on Apr. 25, 2005 inthe names of John R. Martin, Timothy J. Brosnihan, Craig Core, ThomasKieran Nunan, Jason Weigold, and Xin Zhang [practitioner's file2550/A47], which is hereby incorporated herein by reference in itsentirety.

This application may be related to U.S. patent application Ser. No.11/028,249 (now abandoned) entitled Method of Forming a MEMS Device,filed Jan. 3, 2005 in the names of Thomas Kieran Nunan and Timothy J.Brosnihan [practitioner's file 2550/A40], which is hereby incorporatedherein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to micromachined devices, and,more particularly, to micromachined microphones and inertial sensors.

BACKGROUND OF THE INVENTION

Micromachined microphones typically include a thin diaphragm electrodeand a fixed sensing electrode that is positioned alongside the diaphragmelectrode. The diaphragm electrode and the fixed sensing electrode actlike plates of a capacitor. During operation of the microphone, chargesare placed on the diaphragm electrode and the fixed sensing electrode.As the diaphragm electrode vibrates in response to sound waves, thechange in distance between the diaphragm electrode and the fixed sensingelectrode results in capacitance changes that correspond to the soundwaves.

FIG. 1 shows the general structure of a micromachined microphone asknown in the art. Among other things, the micromachined microphoneincludes a diaphragm 102 and a bridge 104. The diaphragm 102 and thebridge 104 act as electrodes for a capacitive circuit. As shown, thebridge 104 may be perforated to allow sound waves to reach the diaphragm102. Alternatively or additionally, sound waves can be made to reach thediaphragm through other channels. In any case, sound waves cause thediaphragm to vibrate, and the vibrations can be sensed as changes incapacitance between the diaphragm 102 and the bridge 104. Themicromachined microphone typically includes a substantial cavity 106behind the diaphragm 102 in order to allow the diaphragm 102 to movefreely.

In a typical micromachined microphone, sound waves reach the diaphragmthrough perforations in the fixed sensing electrode. The size and depthof the perforations can affect the quality of sound reproduction.

SUMMARY OF THE INVENTION

Embodiments of the present invention include micromachined microphonesand micromachined multisensors (including both a microphone and aninertial sensor on a single chip) that can be very small and thin.Certain markets for micromachined devices, such as cellular phones andpersonal digital assistants (PDAs), place considerable value on small,thin components.

In accordance with one aspect of the invention there is provided amethod for producing a micromachined microphone from a wafer having atleast a first silicon layer. The method involves forming at least oneoxide layer on a front side of the first silicon layer, forming aplurality of polysilicon microphone structures including a diaphragm onthe at least one oxide layer, and removing a portion of the at least oneoxide layer underlying the plurality of polysilicon microphonestructures from a back side of the first silicon layer through aplurality of trenches formed through the first silicon layer. Theplurality of trenches allow sound waves to reach the diaphragm from theback side of the first silicon layer. For illustrative purposes, thesound path in this exemplary example is described as reaching thediaphragm from the backside. However, a front side sound path isequivalent and is incorporated in the description of all of the processand design variations described herein.

In certain embodiments of the invention, the at least one oxide layermay be formed on the front side of the first silicon layer by depositinga single oxide layer on the front side of the first silicon layer. Inother embodiments of the invention, the at least one oxide layer may beformed on the front side of the first silicon layer by forming thetrenches through the first silicon layer, depositing a first oxide layercovering the front side of the first silicon layer and lining thetrenches, forming a plurality of sacrificial polysilicon microphonestructures on the first oxide layer, depositing a second oxide layerover the first oxide layer and the sacrificial polysilicon microphonestructures, and removing the sacrificial polysilicon microphonestructures. In certain embodiments of the invention, the plurality ofsacrificial polysilicon structures are formed on the first oxide layerby depositing a polysilicon layer covering the first oxide layer andfilling the lined trenches, and patterning the polysilicon layer to formthe plurality of sacrificial polysilicon microphone structures. XeF₂ isused as an exemplary example of a polysilicon sacrificial etch materialand etch process throughout this description. However, other siliconetchants and etch processes can be used and are incorporated in thedescription of all of the process and design variants described herein.

Before removing the portion of the at least one oxide layer underlyingthe plurality of polysilicon structures, an additional oxide layer maybe formed over the at least one oxide layer and the plurality ofpolysilicon structures, the additional oxide layer may be patterned toexpose a portion of a polysilicon structure and a portion of the firstsilicon layer, and metallic electrodes may be formed to at least theexposed portion of the polysilicon structure and the exposed portion ofthe first silicon layer. At least one metallic bond pad may also beformed at this time. A passivation layer (typically including an oxidelayer covered by a nitride layer) may be formed over the metallicelectrodes. The passivation layer may be patterned to expose at least aportion of an edge of the diaphragm, and a pedestal may be formedbeneath the edge of the diaphragm, for example, by depositing a firstphotoresist layer over the exposed portion of the edge of the diaphragm,patterning the photoresist material to re-expose the portion of the edgeof the diaphragm, removing a portion of oxide beneath the portion of theedge of the diaphragm, and depositing a second photoresist layer forminga pedestal beneath the edge of the diaphragm. Similarly, photoresist maybe patterned over holes in the diaphragm to allow a portion of oxide tobe removed under the diaphragm. A second layer of photoresist at aplurality of these locations forms a plurality of pedestals directlyunder the diaphragm. The pedestal supports the diaphragm during removalof the portion of the at least one oxide layer underlying the pluralityof polysilicon structures. The pedestal is removed after removal of theportion of the at least one oxide layer underlying the plurality ofpolysilicon structures. It should be noted that the described techniquesfor forming pedestals under the diaphragm, removing sacrificialmaterial, and removal of pedestals may be similar or related totechniques described in U.S. Pat. No. 5,314,572 entitled Method forFabricating Microstructures, which is hereby incorporated herein byreference in its entirety.

In certain embodiments of the invention, a multisensor including themicrophone and an inertial sensor may be formed on the same wafer. Theinertial sensor is formed in part by forming a plurality of polysiliconinertial sensor structures during formation of the polysiliconmicrophone structures and removing a portion of the at least one oxidelayer underlying the plurality of polysilicon inertial sensor structuresfrom a back side of the first silicon layer through at least one trenchthrough the first silicon layer during removal of the portion of the atleast one oxide layer underlying the plurality of polysilicon microphonestructures. As with the microphone, a plurality of sacrificialpolysilicon inertial sensor structures may be formed on the first oxidelayer during formation of the plurality of sacrificial polysiliconmicrophone structures, the second oxide layer may be deposited over thesacrificial polysilicon inertial sensor structures, and the sacrificialpolysilicon inertial sensor structures may be removed. The polysiliconlayer is preferably patterned to form the plurality of sacrificialpolysilicon inertial sensor structures during formation of thesacrificial polysilicon microphone structures.

In certain embodiments of the invention, the wafer is an SOI waferfurther including a second silicon layer and an intermediate oxide layerbetween the first silicon layer and the second silicon layer. In thiscase, removing the portion of the at least one oxide layer underlyingthe plurality of polysilicon microphone structures from the back side ofthe first silicon layer may involve removing underlying portions of thesecond silicon layer and the intermediate oxide layer to form a backside cavity and removing the portion of the at least one oxide layerunderlying the plurality of polysilicon microphone structures throughthe back side cavity. In the case of a multisensor, back side cavitiesmay be formed for both the microphone and the inertial sensor, and theportions of the oxide layers underlying the microphone and inertialsensor structures are removed through the back side cavities. A glasslayer may be formed on a back side of the second silicon layer so as tocover and seal the back side cavity of the inertial sensor but not theback side cavity of the microphone.

In accordance with another aspect of the invention there is providedapparatus including a wafer having at least a first silicon layer andincluding a plurality of trenches formed through the first silicon layerand a plurality of polysilicon microphone structures, including adiaphragm, formed above a front side of the first silicon layer. Theplurality of polysilicon microphone structures are formed by depositingat least one oxide layer on the front side of the first silicon layer,forming the polysilicon microphone structures on the at least one oxidelayer, and subsequently removing a portion of the at least one oxidelayer underlying the plurality of polysilicon microphone structures froma back side of the first silicon layer through the plurality oftrenches. The plurality of trenches allow sound waves to reach thediaphragm from the back side of the first silicon layer.

In certain embodiments of the invention, the apparatus may also includea plurality of polysilicon inertial sensor structures formed above thefront side of the first silicon wafer, wherein the plurality ofpolysilicon microphone structures and the plurality of polysiliconinertial sensor structures are formed substantially at the same timeusing substantially the same processes.

In certain embodiments of the invention, the wafer is an SOI waferincluding a second silicon layer and an intermediate oxide layer betweenthe first silicon layer and the second silicon layer, in which caseunderlying portions of the second silicon layer and the intermediateoxide layer are removed to form a back side cavity exposing thetrenches. The trenches may be formed through the first silicon layerthrough the back side cavity.

In accordance with another aspect of the invention there is providedapparatus including a micromachined microphone and a micromachinedinertial sensor formed on the same wafer. The microphone and theinertial sensor include polysilicon structures formed above a front sideof a top silicon layer of the wafer. The top silicon layer includes aplurality of trenches allowing removal of oxide underlying thepolysilicon structures from a back side of the top silicon layer duringproduction and also allowing sound waves to reach a microphone diaphragmfrom the back side.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

The foregoing and advantages of the invention will be appreciated morefully from the following further description thereof with reference tothe accompanying drawings wherein:

FIG. 1 shows the general structure of a micromachined microphone asknown in the art;

FIGS. 2A-2N demonstrate a first exemplary process for forming amicromachined microphone from an SOI wafer in accordance with anembodiment of the present invention;

FIG. 3 shows an exemplary configuration for a device combining amicromachined microphone or multisensor with an IC wafer in accordancewith an embodiment of the present invention;

FIG. 4 shows the general layout of a first exemplary two-axis (X-Y)accelerometer in accordance with an embodiment of the present invention;

FIG. 5 shows the general layout of a second exemplary two-axis (X-Y)accelerometer in accordance with an embodiment of the present invention;

FIG. 6 shows the general layout of an exemplary Z-axis accelerometer inaccordance with an embodiment of the present invention;

FIGS. 7A-7N demonstrate an exemplary process for forming a combinedmicrophone and two-axis accelerometer from an SOI wafer in accordancewith an embodiment of the present invention;

FIGS. 8A-8M demonstrate an exemplary process for forming a combinedmicrophone and three-axis accelerometer from an SOI wafer in accordancewith an embodiment of the present invention; and

FIGS. 9A-9O demonstrate an exemplary process for forming a combinedmicrophone and three-axis accelerometer from a regular silicon wafer inaccordance with an embodiment of the present invention.

Unless noted otherwise, the drawings are not drawn to scale.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

In embodiments of the present invention, a micromachined microphone isformed from a silicon or silicon-on-insulator (SOI) wafer. As known inthe art, a SOI wafer includes a top silicon layer, usually called thedevice layer, an intermediate insulator (oxide) layer, and a bottomsilicon layer that is typically much thicker than the top silicon layer(approximately 650 microns). In this invention, the top layer formed ineither a silicon or a SOI wafer may be approximately 10 microns thick insome embodiments of the invention or much thicker, approximately 50microns thick, in other embodiments. In embodiments of the presentinvention, the fixed sensing electrode (also referred to herein as a“backplate”) is formed from the top silicon layer of the wafer, and thediaphragm is formed so as to be suspended above the top silicon layer.Perforations are formed in the fixed sensing electrode to allow soundwaves to reach the diaphragm from the bottom side of the wafer. An oxidelayer on the back side of the top silicon layer, which may be theinherent oxide layer of a SOI wafer or an oxide layer deposited on asilicon wafer, is used as an etch stop layer for controlling themachining of the fixed sensing electrode. In certain embodiments of theinvention, an inertial sensor, such as a micromachined accelerometer orgyroscope, is formed on the same wafer as the microphone. Forconvenience, such an arrangement may be referred to hereinafter as a“multisensor,” since it includes multiple micromachined sensor elementson a single chip. Provision is made for having the microphone diaphragmopen to air but the inertial sensor hermetically sealed.

A first exemplary process for forming a micromachined microphone from anSOI wafer in accordance with an embodiment of the present invention isdescribed with reference to FIGS. 2A-2N.

In FIG. 2A, trenches are etched through the top silicon layer 202 of ablank SOI wafer into the intermediate oxide layer 204 and optionallythrough to the bottom silicon layer 206. The trenches are then linedwith an oxide material 208. A polysilicon material 210 is then depositedso as to fill the lined trenches and cover the top silicon.

In FIG. 2B, the polysilicon material is patterned and etched to formvarious sacrificial structures 212 that will be removed later.

In FIG. 2C, additional oxide material 214 is deposited. The site of afuture oxide pedestal 216, described below, is highlighted.

In FIG. 2D, features including the microphone diaphragm 218 andsuspension spring 220 are deposited and patterned from a polysiliconmaterial. The diaphragm is typically round, although this is not arequirement of the invention. The diaphragm may be solid or perforated.The gap between the diaphragm and the surrounding polysilicon ispreferably very small so that the sound waves act substantially on oneside of the diaphragm only.

In FIG. 2E, oxide 222 is deposited, and holes 224 are etched. The holesare used for electrodes that make electrical connections to thediaphragm and backplate, as described below.

In FIG. 2F, metal is deposited and patterned in order to form anelectrode 226 for placing electrical charge on the diaphragm, anelectrode 228 for placing electrical charge on the backplate, and aplurality of bond pads 230. There may be electrical connections (notshown) between bond pads 230 and the electrodes 226 and 228.

In FIG. 2G, passivation layers 232 are deposited. The passivation layerstypically include an oxide layer covered by a nitride layer, which is astandard passivation layer used for integrated circuitry. Thepassivation layers 232 are etched at 234 to expose the bond pad 230.

In FIG. 2H, the passivation layers 232 are etched to expose thediaphragm 218.

In FIG. 2I, a photoresist material 236 is deposited and then patternedto expose the future pedestal area 238. The oxide at the future pedestalarea is then removed by etching.

In FIG. 2J, the remaining photoresist material is removed, and thebottom silicon layer 206 is optionally thinned from approximately 650microns to approximately 350 microns by any of several methods includingetching, grinding and polishing.

In FIG. 2K, photoresist material 240 is deposited on the front side ofthe wafer so as to form a photoresist pedestal 242. Photoresist material244 is also deposited on back side of the wafer and patterned to outlinea backside cavity 246. The backside cavity 246 is formed by etching awaya portion of the bottom silicon layer 206 to the intermediate oxidelayer 204. In an exemplary embodiment, the backside cavity 246 afterpackaging is approximately one cubic millimeter in volume.

In FIG. 2L, a portion of the intermediate oxide layer within the cavity246 is removed in order to expose the sacrificial polysiliconstructures.

In FIG. 2M, the sacrificial polysilicon structures are removed,preferably by exposing the polysilicon to XeF₂ gas or another suitablesilicon etchant through the backside cavity 246. It should be noted thatthe XeF₂ gas may remove some of the exposed bottom silicon layer,although this is generally undesirable.

In FIG. 2N, the oxide behind the diaphragm 218 is removed, preferably byplacing in an appropriate liquid. Then, the front side photoresistmaterial 240 (including the pedestal) is removed, preferably in a dryetch (not a liquid). This essentially releases the diaphragm and relatedstructures. It should be noted that the pedestal is used to support thedelicate microphone structures during release and may not be required inall embodiments, particularly if vapor HF is used to remove the oxideinstead of a liquid.

In certain embodiments of the invention, a micromachined microphone andan inertial sensor (such as a gyroscope or accelerometer) are formed onthe same wafer and are integrated into a single chip. The microphone isgenerally open to air in order to allow sound waves to reach themicrophone diaphragm, although the inertial sensor may be hermeticallysealed on the wafer.

FIG. 4 shows the general layout of a first exemplary two-axis (X-Y)accelerometer in accordance with an embodiment of the present invention.The accelerometer includes a frame 402 and a mass 404 that is suspendedfrom the frame by a number of suspension springs 406. The mass includesa number of finger structures that are interdigitated with a number offixed sensing fingers. In this example, there are two sets of fixedsensing fingers 408 and 410 for sensing movement of the mass 404relative to the frame 402 in the X axis and two sets of fixed sensingfingers 412 and 414 for sensing movement of the mass 404 relative to theframe 402 in the Y axis. In the example shown in FIG. 4, the fixedsensing fingers are off-center (i.e., are closer to one mass finger thanto the adjacent mass finger), which allows for differential capacitancemeasurement.

FIG. 5 shows the general layout of a second exemplary two-axis (X-Y)accelerometer in accordance with an embodiment of the present invention.The accelerometer includes a frame 502 and a mass 504 that is suspendedfrom the frame by a number of suspension springs 506. In this example,there are two electrodes 508 and 510 for sensing movement of the mass504 relative to the frame 502 in the X axis and two electrodes 512 and514 for sensing movement of the mass 504 relative to the frame 502 inthe Y axis.

FIG. 6 shows the general layout of an exemplary Z-axis accelerometer inaccordance with an embodiment of the present invention. Theaccelerometer includes a frame 602 and a mass 604 that is suspended fromthe frame by a number of suspension springs 606. In this example, themass 604 is designed to pivot or “teeter-totter” about the springs 606under z-axis acceleration so that there is displacement of the mass outof the plane of the frame/mass. Electrodes (not shown) are positioned todetect such out-of-plane movement of the mass 604.

An exemplary process for forming a combined microphone and two-axisaccelerometer from an SOI wafer is described with reference to FIGS.7A-7N. In order to show both the microphone region and the accelerometerregion of the wafer at each step, the microphone region is shown abovethe accelerometer region, although it is understood that the regions areactually beside one another on the wafer. It should be noted that thisprocess is a variation of the one described above with reference toFIGS. 2A-2N, and these processes could be used to produce just themicromachined microphone, or, for that matter, just the accelerometer.

In FIG. 7A, trenches are etched through the top silicon layer of a SOIwafer in both the microphone region and the accelerometer region.Portions of the intermediate oxide layer beneath the trenches isremoved.

In FIG. 7B, a thermal oxide material is grown. This lines the trenchesand the cavity in the intermediate oxide layer and also covers the toplayer of the wafer.

In FIG. 7C, a polysilicon material is patterned and etched to formvarious sacrificial structures that will be removed later.

In FIG. 7D, additional oxide material is deposited. The site of a futureoxide pedestal, described below, is highlighted.

In FIG. 7E, features including the microphone diaphragm, microphonesuspension springs, and accelerometer electrode are deposited andpatterned from a polysilicon material. The diaphragm is typically round,although this is not a requirement of the invention. The diaphragm maybe solid or perforated. The gap between the diaphragm and thesurrounding polysilicon is preferably very small so that the sound wavesact substantially on one side of the diaphragm only.

In FIG. 7F, oxide is deposited, and holes are etched. The holes are usedfor electrodes to the microphone diaphragm and backplate as well as tothe accelerometer electrode and intermediate oxide layer, as describedbelow.

In FIG. 7G, metal is deposited and patterned in order to form bond padsand electrodes for placing charge on the microphone diaphragm andbackplate as well as on the accelerometer electrode and intermediateoxide layer. There may be electrical connections (not shown) between thebond pads and one or more of the electrodes.

In FIG. 7H, a photoresist material is deposited and patterned. Trenchesare then etched to expose the sacrificial polysilicon. The sacrificialpolysilicon is removed, and the photoresist material is removed.

In FIG. 7I, passivation layers are deposited. The passivation layerstypically include an oxide layer covered by a nitride layer, which is astandard passivation layer used for integrated circuitry. Thepassivation layers are etched to expose the bond pads and the microphonediaphragm.

In FIG. 7J, a photoresist material is deposited about the microphoneregion and then patterned to expose the future pedestal area. The oxideat the future pedestal area is then removed by etching.

In FIG. 7K, the remaining photoresist material is removed, and thebottom silicon layer is optionally thinned from approximately 650microns to approximately 350 microns by any of several methods includingetching, grinding and polishing.

In FIG. 7L, photoresist material is deposited on the front side of thewafer so as to form a photoresist pedestal. Photoresist material is alsodeposited on back side of the wafer and patterned to outline a backsidecavity for the microphone. The backside cavity is partially formed byetching away a portion of the bottom silicon layer to the intermediateoxide layer.

In FIG. 7M, a portion of the intermediate oxide layer within the cavityis removed in order to expose the oxide lining the trenches andunderlying the microphone structures. The oxide behind the diaphragm isthen removed, preferably by placing in an appropriate etchant such asaqueous HF bath.

In FIG. 7N, the front side photoresist material (including the pedestal)is removed, preferably in a dry etch (not a liquid). This essentiallyreleases the diaphragm and related structures. It should be noted thatthe pedestal is used to support the delicate microphone structuresduring release and may not be required in all embodiments, particularlyif vapor HF is used to to remove the oxide instead of a liquid.

An exemplary process for forming a combined microphone and three-axisaccelerometer from an SOI wafer is described with reference to FIGS.8A-8M. In order to show both the microphone region and the accelerometerregion of the wafer at each step, the microphone region is shown abovethe accelerometer region, although it is understood that the regions areactually beside one another on the wafer. It should be noted that thisprocess is a variation of the one described above with reference toFIGS. 7A-7N, and this process could be used to produce just themicromachined microphone, or, for that matter, just the accelerometer.

In FIG. 8A, trenches etched through the top silicon layer of an SOIwafer are lined with nitride and filled with polysilicon.

In FIG. 8B, cap anchors are formed in the accelerometer region bydepositing a layer of oxide material, etching out the anchor locations,and depositing nitride.

In FIG. 8C, a polysilicon material is patterned and etched to formvarious microphone structures, including the diaphragm, as well as a capfor the accelerometer.

In FIG. 8D, oxide is deposited, and holes are etched. The holes are usedfor electrodes to the microphone diaphragm and backplate as well as tothe accelerometer cap and top silicon layer, as described below.

In FIG. 8E, metal is deposited and patterned in order to form bond padsand electrodes for placing charge on the microphone diaphragm andbackplate as well as on the accelerometer cap and top silicon layer.There may be electrical connections (not shown) between the bond padsand one or more of the electrodes.

In FIG. 8F, passivation layers are deposited. The passivation layerstypically include an oxide layer covered by a nitride layer, which is astandard passivation layer used for integrated circuitry. Thepassivation layers are etched to expose the bond pads.

In FIG. 8G, a portion of the passivation layers above the microphonestructures is removed and the oxide over and partially under thepolysilicon structures is removed to form resist pedestal areas.

In FIG. 8H, the bottom silicon layer is optionally thinned fromapproximately 650 microns to approximately 350 microns by any of severalmethods including etching, grinding and polishing, and a layer of oxideis deposited.

In FIG. 8I, a photoresist material is deposited on the front side of thewafer, and the oxide on the back side of the wafer is patterned.

In FIG. 8J, a photoresist material is deposited and patterned on theback side of the wafer, and trenches are etched through the bottomsilicon layer and the intermediate oxide layer of the wafer.

In FIG. 8K, the trenches are etched through the top silicon layer of thewafer to the resist pedestal areas of the microphone region and theoxide underlying the cap of the accelerometer region. The photoresistmaterial is then removed from both the front side and the back side, anda new layer of photoresist material is deposited on the front side forprotection. Cavities are then etched in the back side of the wafer usingthe existing oxide as a hard mask.

In FIG. 8L, the oxide exposed through the cavities is removed,preferably by exposing to HF gas.

In FIG. 8M, borosilicate glass is aligned and anodic bonded to the backside of the wafer. Microphone holes are preferably ultrasonically cut inthe glass prior to bonding. The remaining photoresist material isremoved from the front side of the wafer, thereby releasing themicrophone structures.

An exemplary process for forming a combined microphone and three-axisaccelerometer from a regular silicon wafer is described with referenceto FIGS. 9A-9O. In order to show both the microphone region and theaccelerometer region of the wafer at each step, the microphone region isshown above the accelerometer region, although it is understood that theregions are actually beside one another on the wafer. It should be notedthat this process is a variation of the one described above withreference to FIGS. 8A-8M, and this process could be used to produce justthe micromachined microphone, or, for that matter, just theaccelerometer.

In FIG. 9A, an oxide layer is deposited on the silicon wafer. Aphotoresist material is then deposited and patterned. Trenches are thenetched into the silicon for accelerometer electrodes. The remainingphotoresist and oxide hard mask are then removed. It should be notedthat this step is not required for a microphone-only product.

In FIG. 9B, an oxide layer is grown on both the front side and the backside of the wafer to approximately 1.6 microns. The lined trenches arethen filled with a polysilicon material. It should be noted that thisstep is not required for a microphone-only product.

In FIG. 9C, the remaining oxide is stripped, for example, by exposing toHF gas. Nitride anchor pads are then deposited and patterned on theaccelerometer region. A layer of Novellus oxide is then deposited as asacrificial layer. It should be noted that this step is not required fora microphone-only product.

In FIG. 9D, the oxide layer is etched to expose the nitride pads. Then,a polysilicon material is patterned and etched to form variousmicrophone structures, including the diaphragm, as well as a cap for theaccelerometer.

In FIG. 9E, an oxide material is deposited, and holes are etched. Theholes are used for electrodes to the microphone diaphragm and backplateas well as to the accelerometer cap and silicon layer, as describedbelow.

In FIG. 9F, metal is deposited and patterned in order to form bond padsand electrodes for placing charge on the microphone diaphragm andbackplate as well as on the accelerometer cap and silicon layer. Theremay be electrical connections (not shown) between the bond pads and oneor more of the electrodes.

In FIG. 9G, passivation layers are deposited. The passivation layerstypically include an oxide layer covered by a nitride layer, which is astandard passivation layer used for integrated circuitry. Thepassivation layers are etched to expose the bond pads.

In FIG. 9H, a portion of the passivation layers above the microphonestructures is removed and the oxide over and partially under thepolysilicon structures is removed to form resist pedestal areas.

In FIG. 9I, the silicon wafer is optionally thinned from approximately650 microns to approximately 350 microns by any of several methodsincluding etching, grinding and polishing the back side, and a layer ofoxide is deposited.

In FIG. 9J, a photoresist material is deposited on the front side of thewafer, and the oxide on the back side of the wafer is patterned.

In FIG. 9K, a photoresist material is deposited and patterned on theback side of the wafer, and trenches are etched into the silicon wafer.

In FIG. 9L, the photoresist material is removed from both the front sideand the back side, and a new layer of photoresist material is depositedon the front side for protection. Cavities are then etched in the backside of the wafer using the existing oxide as a hard mask. The trenchesare then further etched through the silicon layer to the resist pedestalareas of the microphone region and the oxide underlying the cap of theaccelerometer region.

In FIG. 9M, the oxide exposed through the cavities is removed,preferably by exposing to HF gas.

In FIG. 9N, the remaining photoresist material is removed from the frontside of the wafer, thereby releasing the microphone structures.

In FIG. 9O, borosilicate glass is aligned and anodic bonded to the backside of the wafer. Microphone holes are preferably ultrasonically cut inthe glass prior to bonding.

A micromachined microphone or multisensor may be assembled with anintegrated circuit (IC) die in a single package. FIG. 3 shows anexemplary configuration for a device combining a micromachinedmicrophone or multisensor with an IC die in a pre-molded plastic packagein accordance with an embodiment of the present invention. The packagecontains a MEMS (Micro-Electro-Mechanical System) die 312 that includesthe micromachined microphone and an integrated circuit (IC) die 314 thatincludes various electronics for processing signals, including thosegenerated by the MEMS die 312. The MEMS die 312 and the IC die 314 aredie attached to the package leadframe 316. After wire bonding, thepackage is sealed, including lid 302 and package body 303. When mountedon a substrate in the embodiment shown, sound waves reach the microphonediaphragm through a downward-facing sound port 306 along a sound path318 that includes spaces between the leadframe 316 and the substrate304. These spaces are formed by the height of the solder pads and,optionally, by passages etched into the bottom surface of the leadframe316. Such passages are typically formed when the leadframes 316 areetched from metal foils by using appropriate patterns in aphotoresist-based process, or other suitable patterning method. A poroushydrophobic filter 308 or other filter may be included, for example, toprotect the microphone diaphragm and other exposed features frommoisture, particulates or other elements. Filter 308 may be laminated toa support fabric to enhance manufacturability and durability. In onealternative embodiment, the substrate 304 could include a hole to allowsound waves to reach the microphone diaphragm. In another alternativeembodiment, lid 302 could include a hole to allow sound waves to reachthe microphone diaphragm through the top of the package. In anotheralternative embodiment, the plastic side walls 303 of the package couldhave a plurality of slots to allow sound waves to reach the microphonediaphragm. In another alternative embodiment, lid 302, downward-facingsound port 306, and substrate 304 could include holes to allow soundwaves to reach the microphone diaphragm from different locations. Inanother alternative embodiment, a MEMS die 312 that includes themicromachined microphone and an integrated circuit (IC) die 314 thatincludes various electronics for processing signals, including thosegenerated by the MEMS die 312 and a MEMS die containing at least onesealed inertial sensor are assembled in a package.

It should be noted that the described techniques for forming microphoneand inertial sensor structures suspended above the front side of the topsilicon layer may be similar or related to techniques described in U.S.patent application Ser. No. 11/028,249 entitled Method of Forming a MEMSDevice, filed Jan. 3, 2005 in the names of Thomas Kieran Nunan andTimothy J. Brosnihan, which is hereby incorporated herein by referencein its entirety.

It should also be noted that the present invention is not limited to anyparticular shape or configuration of microphone diaphragm. Themicrophone may be, for example, round or square, solid or perforated byone or more holes, and/or flat or corrugated. Different diaphragmconfigurations might require different or additional processes fromthose described. For example, additional processes may be required toform holes or corrugations in the diaphragm.

It should also be noted that the described processes are exemplary only.For any particular implementation, fewer, additional, or different stepsor processes may be required. In some cases, materials different thanthose described may be suitable for a particular step or process. Itwould be virtually impossible to describe every combination andpermutation of materials and processes that could be employed in variousembodiments of the invention. Therefore, the present invention isintended to include all such materials and processes including suitablevariations of the materials and processes described.

The present invention may be embodied in other specific forms withoutdeparting from the true scope of the invention. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive.

What is claimed is:
 1. A method for producing a micromachined microphonefrom a wafer having at least a first silicon layer, the methodcomprising: forming trenches through the first silicon layer; depositinga first oxide layer covering the front side of the first silicon layerand lining the trenches; forming a plurality of sacrificial polysiliconmicrophone structures on the first oxide layer; depositing a secondoxide layer over the first oxide layer and the sacrificial polysiliconmicrophone structures; forming a plurality of polysilicon microphonestructures including a diaphragm on the second oxide layer; removing thesacrificial polysilicon microphone structures; and removing a portion ofat least one oxide layer underlying the plurality of polysiliconmicrophone structures from a back side of the first silicon layerthrough the trenches.
 2. The method according to claim 1, whereinforming the plurality of sacrificial polysilicon microphone structureson the first oxide layer comprises: depositing a polysilicon layercovering the first oxide layer and filling the lined trenches; andpatterning the polysilicon layer to form the plurality of sacrificialpolysilicon microphone structures.
 3. The method according to claim 1,further comprising, before removing the portion of the at least oneoxide layer underlying the plurality of polysilicon microphonestructures: forming an additional oxide layer over the second oxidelayer and the plurality of polysilicon microphone structures; patterningthe additional oxide layer to expose a portion of the polysiliconmicrophone structures and a portion of the first silicon layer; formingmetallic electrodes to at least the exposed portion of the polysiliconmicrophone structure and the exposed portion of the first silicon layer.4. The method according to claim 1, further comprising, before removingthe portion of the at least one oxide layer underlying the plurality ofpolysilicon microphone structures: depositing a first photoresist layerover the diaphragm over at least one hole in the diaphragm; patterningthe first photoresist layer to expose a hole in the diaphragm; removinga portion of oxide beneath the hole in the diaphragm; depositing asecond photoresist layer forming a pedestal beneath the hole in thediaphragm, wherein the pedestal supports the diaphragm during removal ofthe portion of the at least one oxide layer underlying the plurality ofpolysilicon microphone structures; and removing the pedestal afterremoving the portion of the at least one oxide layer underlying theplurality of polysilicon microphone structures.
 5. The method accordingto claim 1, further comprising: forming a plurality of inertial sensorstructures during formation of the polysilicon microphone structures. 6.The method according to claim 1, further comprising: forming a pluralityof sacrificial polysilicon inertial sensor structures on the first oxidelayer during formation of the plurality of sacrificial polysiliconmicrophone structures; depositing the second oxide layer over thesacrificial polysilicon inertial sensor structures; and removing thesacrificial polysilicon inertial sensor structures.
 7. The methodaccording to claim 6, further comprising: patterning a polysilicon layerto form the plurality of sacrificial polysilicon inertial sensorstructures during formation of the sacrificial polysilicon microphonestructures.
 8. The method according to claim 1, wherein the waferfurther includes a second silicon layer and an intermediate oxide layerbetween the first silicon layer and the second silicon layer, andwherein removing the portion of the at least one oxide layer underlyingthe plurality of polysilicon microphone structures from the back side ofthe first silicon layer comprises: removing underlying portions of thesecond silicon layer and the intermediate oxide layer to form a backside cavity; and removing the portion of the at least one oxide layerunderlying the plurality of polysilicon microphone structures throughthe back side cavity.
 9. The method according to claim 5, wherein thewafer further includes a second silicon layer and an intermediate oxidelayer between the first silicon layer and the second silicon layer, andwherein removing the portion of the at least one oxide layer underlyingthe plurality of polysilicon microphone structures from the back side ofthe first silicon layer comprises: removing underlying portions of thesecond silicon layer and the at least one intermediate oxide layer toform back side cavities for the polysilicon microphone structures andthe inertial sensor structures; and removing the portions of the atleast one intermediate material underlying the polysilicon microphonestructures and inertial sensor structures through the back sidecavities.
 10. The method according to claim 9, further comprising:forming a glass layer on a back side of the second silicon layer, theglass layer covering and sealing the back side cavity of the inertialsensor structures but not covering the back side cavity of thepolysilicon microphone structures.
 11. The method according to claim 1,wherein the plurality of trenches allow sound waves to reach thediaphragm from the back side of the first silicon layer.
 12. The methodaccording to claim 1, wherein the first silicon layer is a device layerof an SOI wafer.
 13. The method according to claim 12, wherein the SOIwafer includes an intermediate oxide layer, and wherein forming thetrenches comprises etching through the device layer into theintermediate oxide layer.
 14. The method according to claim 13, whereinthe SOI wafer includes a bottom silicon layer, such that theintermediate oxide layer is between the device layer and the bottomsilicon layer, and wherein forming the trenches comprises etchingthrough the bottom silicon layer.
 15. The method according to claim 2,further comprising: patterning the polysilicon layer to form a pluralityof sacrificial polysilicon inertial sensor structures during formationof the sacrificial polysilicon microphone structures.
 16. The methodaccording to claim 1 wherein the first silicon layer comprises a fixedsensing electrode and wherein forming the trenches comprises formingperforations through the fixed sensing electrode.
 17. A method forproducing a micromachined microphone from a wafer having at least afirst silicon layer, the method comprising: forming at least one oxidelayer on a front side of the first silicon layer; forming a plurality ofpolysilicon microphone structures including a diaphragm on at least oneoxide layer; forming an additional oxide layer over the at least oneoxide layer and the plurality of polysilicon microphone structures;patterning the additional oxide layer to expose a portion of apolysilicon structure and a portion of the first silicon layer; formingmetallic electrodes to at least the exposed portion of the polysiliconstructure and the exposed portion of the first silicon layer; andremoving a portion of at least one oxide layer underlying the pluralityof polysilicon microphone structures from a back side of the firstsilicon layer through a plurality of trenches formed through the firstsilicon layer.
 18. The method according to claim 17, further comprisingforming the trenches through the first silicon layer.
 19. The methodaccording to claim 18 wherein the first silicon layer comprises a fixedsensing electrode and wherein forming the trenches comprises formingperforations through the fixed sensing electrode.
 20. The methodaccording to claim 19 wherein the first silicon layer is a device layerof an SOI wafer, and wherein the SOI wafer further includes anintermediate oxide layer and a bottom silicon layer such that theintermediate oxide layer is between the device layer and the bottomsilicon layer, and wherein forming the trenches further comprisesetching through the intermediate oxide layer.